Compliance is key for protecting the public

The CIPHE takes its responsibilities to the public seriously, and this includes helping our members to stay up to date with regulations, guidance and best practice, particularly where customers’ safety is in question. We work with industry, including other trade organisations and regulatory bodies, to keep you informed of issues that may affect your work.

Recently, this ongoing work has led to the production of a general guidance document about the key points of part G3 of the building regulations. The CIPHE was approached by the Hot Water Association (HWA) which had heard from some of its own members that components were being replaced in heat pump systems that would not only compromise their safe operation, but also invalidate their WRAS/KIWA approval. The product of this is the new CIPHE Position Paper: The Components of a Domestic Hot Water System.

Overview

This guidance note is intended to assist CIPHE members and industry professionals in maintaining compliance with Building Regulations Approved Document G3 and in promoting best practice across the domestic hot water (DHW) sector.

Modern domestic hot water systems are predominantly unvented, operating at mains pressure and incorporating multiple layers of hydraulic and thermal protection. While traditional vented cylinders still exist, G3 mandates safety and control measures that make understanding each component essential for all qualified installers.

Temperature and pressure relief valve (T&PR Valve)

The T&PR valve is the system’s final line of defence. It discharges safely if:

• temperature rises to 90-95°C, or
• pressure exceeds the valve’s set point (typically 6-8 bar).

G3 requires this valve to be factory-fitted. During maintenance, the installer must manually operate it to verify opening and reseating. The discharge pipework – via the tundish—must be correctly routed to a safe outlet and capable of handling full flow.

Regulation reference: Building Regulations Approved Document G3:2015 + A1 (2020)

Standard: BS EN 1490 — Combination temperature and pressure relief valves

Combined inlet control group

Typically supplied by the cylinder manufacturer, this assembly incorporates:

• Pressure reducing valve (PRV): stabilises mains to 3-3.5 bar.
• Expansion valve: opens if expansion control fails.
• Check valve: prevents backflow to the cold main (Water Supply (Water Fittings) Regulations 1999).
• Strainer: captures debris – must be cleaned annually.

All components are factory-set and non-adjustable. Substituting parts can invalidate system approvals and warranties.

Standard: BS EN 1567 (PRVs), WRAS Information & Guidance Note 9-02-05

Expansion vessel

Heating water increases its volume; this must be contained within the system. Where the cylinder lacks an integral air gap, a potable expansion vessel is used.

Constructed from WRAS-approved materials. Pre-charge pressure ≈ 3 bar; check annually. Distinct from heating vessels (usually red).

Continuous dripping from the expansion relief indicates potential pre-charge loss or membrane failure and must be investigated.

Standard: BS EN 13831 — Closed expansion vessels for water systems

Thermal controls

Three independent temperature-control stages are required:

1. Thermostat — maintains storage temperature at ~60°C.
2. Thermal cut-out — non-self-resetting, preset 15-20°C above thermostat.
3. T&PR valve — final safeguard.

For indirect cylinders, the thermostat and cut-out must control a spring-return motorised valve isolating the primary circuit on overheat. Power-on/off (Momo) valves are not permitted under G3.

Regulation reference: G3 Section 3.62 – 3.65

Standard: BS EN 60335-2-73 — Safety of electric storage water heaters

Immersion heater

All modern cylinders incorporate an immersion heater for standby operation or renewable integration (eg PV-diversion). Each unit must include its own thermostat and cut-out.

For PV-linked systems, storage temperatures may reach 70-75°C; fit a tempering valve downstream to maintain safe delivery temperatures.

Isolating valves

The Water Regulations require isolating valves on:

• the cold-water inlet to the DHW system, and

• individual outlets where maintenance access is needed.

Regulation reference: Water Supply (Water Fittings) Regulations 1999 — Schedule 2 para 18

Thermostatic mixing valves (TMVs)

Under G3, TMVs are mandatory at outlets allowing full-body immersion (baths and showers). They limit delivery to 48°C, preventing scalding.

Healthcare premises require TMVs on all hot outlets per HTM 04-01.

Standard: BS EN 1111 and BS EN 1287 — Thermostatic mixing valves

Tempering valve

A system-wide tempering valve blends cylinder outlet water with cold supply to approximately 55°C for distribution. This enhances safety, energy efficiency and system capacity, particularly in solar-assisted or PV-boosted systems.

Tundish

The tundish provides an air gap and visible discharge path for safety-valve outlets. It must be installed within the prescribed 600mm of the relief valve outlet and terminate via visible pipework to a safe drain.

While both T&PR and expansion relief can share a tundish, CIPHE recommends separate fittings for clear fault diagnosis.

Standard: BS 853 — Specification for discharge pipework serving unvented hot water storage systems

Conclusion

Each component within an unvented domestic hot water system performs a specific and regulated function. Correct installation, annual inspection and the use of manufacturer-approved parts are essential to maintain safety, compliance and performance.

By adhering to Building Regulations G3, the Water Regulations 1999, and relevant British Standards, installers uphold the integrity and safety of every domestic hot water system in the UK.

Following this guidance, installers not only ensure safety and compliance but also demonstrate the professional competence expected.